VAVETs have become an interesting area of research since vehicles can be equipped with sensors, processing and communication devices. As a result, various life changing application emerged in different areas such as safety and public services. VANETs are considered as a subclass of Ad hoc networks. However, they have special characteristics that differentiates them such as QoS requirements, privacy, safety and high mobility of nodes. This paper discusses VANET's special characteristics and explain why VANETs are considered a subcategory of ad hoc networks. Also, a categorization of VANETs architectures is presented in this study. Additionally, the importance and needs of VANETs along with their applications are presented in this study. Furthermore various routing protocols proposed for VANETs are studied and a categorization for these protocols is proposed in this study.